Subas Dhakal

Statistical Mechanics of Bend Flexoelectricity and the Twist-Bend Phase in Bent-Core Liquid Crystals

We develop a Landau theory for bend flexoelectricity in liquid crystals of bent-core molecules. In the nematic phase of the model, the bend flexoelectric coefficient increases as we reduce the temperature toward the nematic to polar phase transition. At this critical point, there is a second-order transition from high-temperature uniform nematic phase to low-temperature nonuniform polar phase composed of twist-bend or splay-bend deformations. To test the predictions of Landau theory, we perform Monte Carlo simulations to find the director and polarization configurations as functions of temperature, applied electric field, and interaction parameters.

Topology, length scales, and energetics of surfactant micelles

We study the morphology, energetics, and kinetics of a self-associating model cationic surfactant in water using large-scale coarse-grained molecular dynamics simulations over time scales that allow for probing micelle recombination dynamics. We develop an algorithm to track micelle contours and quantify various microstructural features such as contour length distribution, persistence length, and mesh size. We predict reliably the end-cap energy and recombination time of micelles, directly from molecular simulations for the first time. We further consider the variation of solution viscosity as a function of salt concentration and show that branched and multiconnected structures govern the experimentally observed anomalous dependence of zero-shear viscosity on salt concentration. Overall, simulation predictions are in good agreement with experiments.

Chirality and biaxiality in cholesteric liquid crystals.

We investigate the statistical mechanics of chirality and biaxiality in liquid crystals through a variety of theoretical approaches, including Monte Carlo simulations, lattice mean-field theory, and Landau theory. All of these calculations show that there is an important interaction between cholesteric twist and biaxial order: The twist acts as a field on the biaxial order, and conversely, the biaxial order increases the twist, that is, reduces the pitch. We model the behavior of chiral biaxial liquid crystals as a function of temperature and discuss how the predictions can be tested in experiments.

Statistical Mechanics of Splay Flexoelectricity in Nematic Liquid Crystals

We develop a lattice model for the splay flexoelectric effect in nematic liquid crystals. In this model, each lattice site has a spin representing the local molecular orientation, and the interaction between neighboring spins represents pear-shaped molecules with shape polarity. We perform Monte Carlo simulations and mean-field calculations to find the behavior as a function of interaction parameters, temperature, and applied electric field. The resulting phase diagram has three phases: isotropic, nematic, and polar. In the nematic phase, there is a large splay flexoelectric effect, which diverges as the system approaches the transition to the polar phase. These results show that flexoelectricity can be a statistical phenomenon associated with the onset of polar order.

Structure and rheology of self-assembled aqueous suspensions of nanoparticles and wormlike micelles

Abstract We present a systematic molecular dynamics (MD) simulation study of the structure and rheology of solutions consisting of cationic micelles and negatively charged nanoparticles (NPs) in the presence of a hydrotropic salt, namely, sodium salicylate. The addition of NPs to wormlike micelle (WLM) solutions results in the formation of electrostatically stabilised NP-micelle junctions, leading to a significant enhancement in the viscosity of the mixture due to effective lengthening of micelle clusters. A monotonic increase in zero-shear viscosity is observed as the NP volume fraction is increased. Branched micelles form at sufficiently large salt concentrations. Sliding motion of micelle branches along the contour of a wormlike chain provides an additional mechanism for stress relaxation. Hence, branch formation induces a non-monotonic variation in the solution viscosity as a function of the salt concentration. Reverse non-equilibrium MD simulations were performed to study the effect of uniform and steady shear flow on the viscosity of the WLM-NP mixtures. Beyond a critical shear rate, flow-induced anisotropy, which is quantified by an orientational order parameter, manifests as viscoelastic rheological behaviour. Specifically, at higher NP volume fractions and shear rates that exceed the inverse of a characteristic structure relaxation time, flow-alignment of the microstructure causes pronounced shear thinning.

Anomalous diffusion and stress relaxation in surfactant micelles

We investigate the mechanisms of anomalous diffusion in cationic surfactant micelles using molecular dynamics simulations in the presence of explicit salt and solvent-mediated interactions. Simulations show that when the counterion density increases, saddle-shaped branched interfaces manifest. In experiments, branched structures exhibit lower viscosity as compared to linear and wormlike micelles. This has long been attributed to stress relaxation arising from the sliding motion of branches along the main chain. Our simulations reveal a mechanism of branch motion resulting from an enhanced counterion condensation at the branched interfaces and provide quantitative evidence of stress relaxation facilitated by branched sliding. Furthermore, depending on the surfactant and salt concentrations, which in turn determine the microstructure, we observe normal, subdiffusive, and superdiffusive motions of surfactants. Specifically, superdiffusive behavior is associated with branch sliding, breakage and recombination of micelle fragments, as well as constraint release in entangled systems.